System and method based on multi-source deposition for fabricating perovskite film

ABSTRACT

A system and method for fabricating a perovskite film is provided, the system including a substrate stage configured to rotate around its central axis at a rotation speed, a first set of evaporation units, each coupled to the side section or the bottom section of the chamber, a second set of evaporation units coupled to the bottom section, and a shield defining two or more zones having respective horizontal cross-sectional areas, which are open and facing the substrate, designated for the two or more evaporation units in the second set. The resultant perovskite film includes multiple unit layers, wherein each unit layer is formed by one rotation of the substrate stage, and the composition and thickness of the unit layer are controlled by adjusting at least the evaporation rates, the rotation speed and the horizontal cross-sectional areas.

TECHNICAL FIELD

The present invention relates to a system and method based onmulti-source deposition for fabricating perovskite films.

BACKGROUND ART

A solar cell (also called a photovoltaic cell) is an electrical devicethat converts solar energy directly into electricity by usingsemiconductors that exhibit the photovoltaic effect. Solar photovoltaicsis now, after hydro and wind power, the third most important renewableenergy source in terms of globally installed capacity. Constructions ofthese solar cells are based around the concept of a p-n junction,wherein photons from the solar radiation are converted intoelectron-hole pairs. Examples of semiconductors used for commercialsolar cells include monocrystalline silicon, polycrystalline silicon,amorphous silicon, cadmium telluride, and copper indium galliumdiselenide. Solar cell energy conversion efficiencies for commerciallyavailable cells are currently reported to be around 14-22%.

High conversion efficiency, long-term stability and low-cost fabricationare essential for commercialization of solar cells. For this reason, awide variety of materials have been researched for the purpose ofreplacing conventional semiconductors in solar cells. For example, thesolar cell technology using organic semiconductors is relatively new,wherein these cells may be processed from liquid solution, potentiallyleading to inexpensive, large scale production. Besides organicmaterials, organometal halide perovskites, for example, CH₃NH₃PbX₃,where X═Cl, Br, I, or a combination thereof, have recently emerged as apromising material for the next generation of high efficiency, low costsolar technology. It has been reported that these synthetic perovskitesexhibit high charge carrier mobility and lifetime that allowlight-generated electrons and holes to move far enough to be extractedas current, instead of losing their energy as heat within the cell.These synthetic perovskites can be fabricated by using the samethin-film manufacturing techniques as those used for organic solarcells, such as solution processing, vacuum evaporation techniques, etc.

Recent reports have indicated that this class of materials, i.e.,organometal halide perovskites, have potential for high-performancesemiconducting media in optoelectronic devices as well. In particular,some perovskites are known to exhibit strong photoluminescenceproperties, making them attractive candidates for use in light-emittingdiodes (LEDs). Additionally, it has been reported that the perovskitesalso exhibit coherent light emission properties, hence opticalamplification properties, suitable for use in electrically drivenlasers.

However, to date, it has been difficult to obtain highly uniformperovskite films with good stoichiometry based on the existingfabrication techniques. Furthermore, these existing techniques are notrobust enough for fabricating perovskite films for bandgap engineering,multi-junction or Tandem cell fabrication, doping control,heterostructure construction, and other advanced solar cell andoptoelectronics applications. In view of ever increasing needs for lowcost fabrication techniques of high-performance devices, a newfabrication technique is desired for producing highly uniform perovskitefilms suitable for solar cell and optoelectronics applications includingLEDs and lasers.

CITATION LIST Non Patent Literature

-   NPL1: Guichuan Xing et al., Low-temperature solution-processed    wavelength-tunable perovskites for lasing. Nature Materials Vol. 13,    476-480 (March, 2014).-   NPL2: Zhi-Kuan Tan et al., Bright light-emitting diodes based on    organometal halide perovskite. Nature Nanotechnology Vol. 9, 687-692    (September, 2014).-   NPL3: Wan-Jian Yin et al., Unique properties of halide perovskite as    possible origins of the superior solar cell performance. Advanced    Materials, DOI:10.1002/adma.201306281 (2014).-   NPL4: Giles E. Eperon et al., Formamidinium lead trihalide: a    broadly tunable perovskite for efficient planar heterojunction solar    cells. Energy Environ. Sci. 7, 982-988 (2014).-   NPL5: Yuhei Ogomi et al., CH₃NH₃Sn_(x)Pb_((1-x))I₃ perovskite solar    cells covering up to 1060 nm. J. Phys. Chem. Lett. 5, 1004-1011    (2014).-   NPL6: Hyosung Choi et al., Cesium-doped methylammonium lead iodide    perovskite light absorber for hybrid solar cells. Nano Energy 7,    80-85 (2014).-   NPL7: Mingzhen Liu et al., Efficient planar heterojunction    perovskite solar cells by vapour deposition. Nature Vol. 501,    395-398 (2013).-   NPL8: Nam-Gyu Park, Organometal perovskite light absorbers toward a    20% efficiency low-cost solid-state mesoscopic solar cell. J. Phys.    Chem. Lett. 4, 2423-2429 (2013).-   NPL9: Julian Burschka et al., Sequential deposition as a route to    high-performance perovskite-sensitized solar cells. Nature Vol. 499,    316-320 (July, 2013).

SUMMARY

A system and method for fabricating a perovskite film is provided, thesystem including a substrate stage configured to rotate around itscentral axis at a rotation speed, a first set of evaporation units, eachcoupled to the side section or the bottom section of the chamber, asecond set of evaporation units coupled to the bottom section, and ashield defining two or more zones having respective horizontalcross-sectional areas, which are open and facing the substrate,designated for the two or more evaporation units in the second set. Theresultant perovskite film includes multiple unit layers, wherein eachunit layer is formed by one rotation of the substrate stage, and thecomposition and thickness of the unit layer are controlled by adjustingat least the evaporation rates, the rotation speed and the horizontalcross-sectional areas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a unit cell of an organometal halide perovskitestructure. (See NPL4)

FIG. 2 illustrates an example of the system configuration forfabricating a perovskite film according to an embodiment.

FIGS. 3A and 3B illustrate examples of the evaporation unit in the firstset coupled to the side section of the chamber.

FIGS. 4A-4C illustrate an example of the evaporation unit in the secondset coupled to the bottom section of the chamber.

FIG. 5 illustrates another example of the system configuration forfabricating a perovskite film according to an embodiment

FIG. 6 is a flowchart illustrating an example of the fabrication processof a perovskite film using the present system.

FIG. 7 is a flowchart illustrating an example of a modified process toinclude the growth of the wetting layer prior to the perovskite filmgrowth using the present system.

FIG. 8 is a flowchart illustrating an example of the fabrication processof a perovskite heterostructure using the present system.

FIG. 9 is a plot of the J-V curve representing the photovoltaic devicecharacterization of a solar cell including the chloride iodideperovskite film, CH₃NH₃PbI_(3-X)Cl_(X) film, grown by the presentfabrication system and method.

FIG. 10 is a plot of the measured absorbance of the CH₃NH₃PbI_(3X)Cl_(X)film.

FIGS. 11A and 11B are photos showing the atomic force microscopy (AFM)images of the CH₃NH₃PbI_(3X)Cl_(X) film.

FIG. 12 is a plot showing the X-ray diffraction (XRD) spectrum of theCH₃NH₃PbI_(3X)Cl_(X) film.

FIGS. 13A and 13B are photos showing the AFM images of the structure ofSi/100 nm perovskite/50 nm spiro-MeOTAD, and the AFM image of thestructure of Si/10 nm PbCl₂/100 nm perovskite/50 nm spiro-MeOTAD,respectively.

DESCRIPTION OF EMBODIMENTS

In view of ever increasing needs for low cost fabrication techniques ofhigh-performance devices, this document describes a new fabricationsystem and method for producing highly crystalline, substantiallyuniform perovskite films for a wide variety of applications, such asbandgap engineering, multi-junction or Tandem cell fabrication, dopingcontrol, heterostructure construction, and so on, especially suitablefor solar cell and optoelectronics applications including LEDs andlasers. The present fabrication method may be regarded as a multi-sourcemolecular-layer deposition, wherein multiple evaporators containingsource materials are configured in a chamber, and multiple growthparameters are used to control the deposition to fabricate a perovskitefilm with a desired profile suitable for target applications. Here, thesource materials include halide materials such as PbCl₂, PbBr₂, PbI₂,SnCl₂, SnBr₂, SnI₂ and the like, and methylammonium (MA=CH₃NH₃ ⁺)compounds such as CH₃NH₃Cl, CH₃NH₃Br, CH₃NH₃I, and the like. In placeof, or in a combination with the MA compound, a formamidinium(FA=HC(NH₂)₂ ⁺) compound can also be used. FIG. 1 illustrates a unitcell of an organometal halide perovskite structure suitable for use fora light absorber. (See NPL4.) This is the orthorhombic structure havingthe general ABX 3 structure, in which an organic element, MA or FA,occupy each site A; a metal element, Pb²⁺ or Sn²⁺, occupy each site B;and a halogen element, Cl⁻, I⁻ or Br, occupy each site X. In thisdocument, AX represents a compound having an organic element MA or FAfor the A-cation combined with a halogen element Cl, I or Br for theX-anion; BX₂ represents a halide material having a metal element Pb orSn for the B-cation combined with a halogen element Cl, I or Br for theX-anion. Examples and implementations according to the present systemand method are described below with reference to accompanying drawings.

FIG. 2 illustrates an example of the system configuration forfabricating a perovskite film according to an embodiment. The systemincludes a chamber 200 coupled to necessary parts. The chamber 200 inthis example has a shape of substantially a hollow cube; however, theshape can be of substantially a hollow cylinder or any other closedhollow structure having a side section along the vertical direction andtop and bottom sections along the horizontal direction. Each section hasan internal surface and an external surface. In FIG. 2, only theinternal surfaces of the chamber 200 are illustrated. The chamber 200 iscoupled to a pump unit 204 for generating near vacuum in the chamber 200for the deposition process. Examples of the pump unit 204 include aturbo molecular pump. The pump unit 204 in FIG. 2 is coupled to the sidesection of the chamber 200 so as to have the pumping directionsubstantially along the horizontal direction along a substrate stage208. As explained later, this configuration allows for the flow of theAX vapor to circulate in the chamber 200 effectively so as to facilitatethe chemical-reaction type of deposition on a substrate. The substratestage 208 is coupled to the top section of the chamber 200 andconfigured to have a large stage surface facing downward for a substrateto be staged facing downward. The area of the stage surface isconfigured to accommodate a large-size substrate or multiple substrates.In this document, a term “substrate” is collectively used for onelarge-size substrate as well as for multiple separate substrates thatare placed on the substrate stage 208. Examples of substrate materialsinclude: fluorine-dope tin oxide (FTO) glass with electron collectionlayer (ECL) such as TiO₂ compact layer or ZnO thin film; indium tinoxide (ITO) glass with hole collection layer (HCL) such as PEDOT:PSS andNiO, where PEDOT:PSS stands for poly(3,4-ethylenedioxythiophene)polystyrene sulfonate; flexible substrate, such as polyethyleneterephthalate, with either ECL or HCL. As explained later, a thinwetting layer can be added to any of the above substrate for enhancingthe quality of a subsequently deposited perovskite film. The temperatureof the substrate stage 208 can be controlled to provide uniform coolingor heating to the one or more substrates, ranging from −10° C. up to350° C., for example. The substrate stage 208 is coupled to a motor 212,which drives to rotate the substrate stage 208 around the central axisduring deposition. The rotation speed of the substrate stage 208 in theunit of Hz, for example, together with other parameters, can bepredetermined based on desired deposition rates and film composition.

In FIG. 2, a first evaporation unit 216 is coupled to the side sectionof the chamber 200, and is configured for generating the AX vapor in thechamber 200. The first evaporation unit 216 may be configured to includea crucible to contain the AX compound in the form of powder, which canbe heated to generate the AX vapor. In the example illustrated in FIG.2, only one evaporation unit coupled to the side section of the chamber200 for generating the AX vapor is shown. However, one or moreevaporation units may be coupled to the side section of the chamber 200for generating one or more types of vapors, respectively. Thus, thepresent system includes a first set of evaporation units comprising oneor more evaporation units, each coupled to the side section of thechamber 200, wherein at least one evaporation unit in the first set isconfigured to generate the AX vapor. As explained later, anotherevaporation unit in the first set may be used to generate vapor of amaterial different from the AX material, such as a volatile dopantmaterial. For example, one evaporation unit in the first set maygenerate the MACl vapor, and another evaporation unit in the first setmay generate the FABr vapor. In another example, one evaporation unit inthe first set may generate the MAI vapor, and another evaporation unitin the first set may generate the vapor of a volatile dopant material.Yet in another example, the first set may have only one evaporation unitfor generating the FAI vapor.

In FIG. 2, a second evaporation unit 220 is coupled to the bottomsection of the chamber 200, and is configured for generating the BX₂vapor in the chamber 200. The second evaporation unit 220 may beconfigured to include a crucible to contain the BX₂ compound in the formof powder, which can be heated to generate the BX₂ vapor. In the exampleillustrated in FIG. 2, four evaporation units coupled to the bottomsection of the chamber 200 are shown. For the purpose of fabricatingmulti-profile films such as a heterostructure or a multi-junctionstructure, the present system may be configured to have a second set ofevaporation units comprising two or more evaporation units coupled tothe bottom section of the chamber 200, wherein at least one evaporationunit in the second set is configured to generate the BX₂ vapor. Asexplained later, another evaporation unit in the second set may be usedto generate vapor of a material different from the BX₂ material, such asa non-volatile dopant material. For example, one evaporation unit in thesecond set may contain PbCl₂, and another evaporation unit may containSnI₂. In another example, one evaporation unit may contain PbBr₂, andanother evaporation unit may contain a non-volatile dopant material.

A shield 224 is structured to separate the two or more evaporation unitsin the second set coupled to the bottom section of the chamber 200,defining individual zones corresponding to these evaporation units. Inthe example of FIG. 2, where four evaporation units in the second setare provided, the shield 224 includes four rectangular plates definingfour zones for the four evaporation units, respectively. The shield 224reduces thermal interference between different types of vapors, enablingindependent evaporation control of different compounds in the respectiveevaporation units. The plate structure of the shield 224 is configuredto be vertically upright to define the individual zones with apredetermined height in the chamber 200 for giving rise to independentdeposition behaviors. The zone designated for each of the evaporationunits in the second set is configured to have a horizontalcross-sectional area that is open and facing the substrate surface. Thehorizontal cross-sectional areas of the separate zones, together withthe rotation speed of the substrate stage 208, can be predeterminedbased on desired deposition rates and growth composition. The horizontalcross-sectional areas can be easily adjusted by repositioning the shield224. Instead of a plate structure, a chimney-like structure having ashape such as a cylinder, a funnel, etc. may be configured to shieldeach evaporation unit from the others in the second set by designating acorresponding zone.

A shutter 228 is provided below the substrate stage 208, and is attachedwith a rod 232, which is used to move the shutter 228 to expose or coverthe substrate stage 208. A push-pull linear motion device, for example,may be coupled to the rod 232 to provide longitudinal motion of theshutter 228 along the axis of the rod 232. Alternatively, the shutter228, the rod 232 and their peripheral parts may be configured to controlthe exposing and covering of the substrate stage 208 based on rotationalmotion around a vertical axis, or any other suitable motion. The shutter228 can be used to start or interrupt the deposition of the vapors ontothe substrate from the evaporation units in the second set coupled tothe bottom section of the chamber 200. A second shutter, which is foruse as an evaporation shutter 236, may be provided for the evaporationunit 220 in the second set. In the example illustrated in FIG. 2, eachof the four evaporation units 220 coupled to the bottom section of thechamber 200 has a corresponding evaporation shutter 236 above it. In thepresent system, two or more evaporation shutters such as 236 may beprovided corresponding to two or more evaporation units in the secondset, respectively. Adjustment of the evaporation shutter 236 iscontrolled by using a rod attached to the shutter 236 via longitudinal,rotational or any other suitable motion by using the rod. Eachevaporation shutter 236 can be used for quick switching on/off of thevapor coming from the evaporation unit 220 into active deposition.

A first monitor 240 is provided near the opening of the evaporation unit216 in the first set coupled to the side section of the chamber 200 tomonitor the evaporation rate of the AX compound, for example. A sensingpart of the first monitor 240 may be configured to face upward so as tomonitor the AX vapor in the chamber rather than the AX vapor flux justcoming out of the evaporation unit 216 in the first set. A secondmonitor 244 is provided above the evaporation unit 220 in the second setcoupled to the bottom section of the chamber 200 and below thecorresponding shutter 236, to monitor the evaporation rate of the BX₂compound, for example. The second monitor 244 may be placed off thecenter of the evaporation unit 220 so as not to block the evaporatedvapor during deposition. Each of the one or more evaporation units 216in the first set coupled to the side section of the chamber 200 as wellas each of the two or more evaporation units 220 in the second setcoupled to the bottom section of the chamber 200 can be provided with amonitor to individually monitor the evaporation rates of the sourcematerials. Examples of these monitors include a quartz crystal thicknessmonitor. The deposition rate and the film thickness in situ can beestimated based on the monitored evaporation rate using the toolingfactor calculation, for example. In this calculation, the ratio betweenthe measured film thickness and the indicated film thickness (asindicated by the monitored evaporation rate) is obtained during a trialrun; thereafter, the ratio can be used to obtain the in situ filmthickness by factoring in the evaporation rate during deposition asobserved by the monitor.

Under certain conditions, the vapor of a volatile material such as AXbecomes kinetically over-active and too much vapor may flow into thepump 204, degrading the internal surface of the pump 204, therebydecreasing the lifetime and/or increasing repair or maintenance cost. Tocircumvent this problem, a filter 250 is detachably attached at theopening of the pump 204 to the chamber 200. The filter may be made of afine fabric mesh held by two concentric ring-shaped frames, asillustrated in FIG. 2. The entire filter 250 or the fabric mesh can bereplaced as needed.

FIGS. 3A and 3B illustrate examples of the evaporation unit 216 in FIG.2. Each example is illustrated as a configuration with respect to theinternal side surface of the chamber 200. FIG. 3A illustrates a cellevaporator, such as a conventional Knudsen cell evaporator, having acell 304 accommodating a crucible 308, in which the powder of the sourcematerial 312 can be contained. A heating element 316 is provided to heatthe crucible 308, hence the powder 312, to generate its vapor. Thetemperature of the heating element 316 is controlled to adjust theevaporation rate of the source material. This cell evaporator mayinclude also an evaporation shutter 320 to control the vapor flux. Thecell evaporator of FIG. 3A is provided at an angle with respect to theinternal side surface of the chamber 200, wherein the angle can bepredetermined so that the vapor is outputted efficiently. Theevaporation shutter 320 is provided to control the flux into the chamber200 to avoid the strong flux of the vapor hitting directly the substratemounted on the substrate stage 208. It is thus preferable that theevaporation shutter 320 is adjusted to be at a position, which allowsfor the vapor to output from the evaporator efficiently and yet preventsthe vapor from directly hitting the substrate during deposition, forenhancing uniformity of the resultant film. FIG. 3B illustrates anotherexample of the evaporation unit 216 in the FIG. 2, which includes anampule 324 to contain the powder of the source material 328, and aheating element 332 provided to heat the ampule 324, hence the powder328, to generate its vapor. The temperature of the heating element 332is controlled to adjust the evaporation rate of the source material. Theevaporation unit illustrated in FIG. 3B further includes a duct 336 toguide the vapor flux into the chamber 200, and is positioned at an anglewith respect to the internal side surface of the chamber 200, whereinthe angle can be predetermined so that the vapor is outputtedefficiently. Alternatively, the duct 336 may be configured to have anextending section to output the vapor in a desired direction in thechamber 200. The duct 336 is coupled to an evaporation shutter 340 toopen or close the duct 336 for controlling the vapor flow in a simplebut timely manner. The shutter 340 is in the form of a valve in thepresent example. In the case where the evaporation unit of FIG. 3B isused for generating vapor of a dopant material, the dopant material iscontained in the ampule 324, and the evaporation shutter 340 such as avalve can be used to control the doping profile in the resultant film.Referring back to FIG. 2, the pump unit 204 is coupled to the sidesection of the chamber 200 so as to have the pumping directionsubstantially along the horizontal direction. The present systemconfiguration having both the pump unit 204 and the evaporation unit 216coupled to the side section allows for the vapor flow of the sourcematerial, such as AX, to circulate in the chamber 200 effectively,thereby facilitating the chemical-reaction type of deposition on thesubstrate.

FIGS. 4A-4C illustrate an example of the evaporation unit 220 in FIG. 2.FIG. 4A illustrates a cross-sectional side view, wherein the evaporationunit 220 includes a crucible 404 to contain the powder of the sourcematerial 408, such as BX₂, and a heating element 412 to heat thecrucible 404, hence the powder 408, to generate its vapor. Two endterminals of the heating element 412 are held by two electricfeedthroughs 416, respectively, to communicate with the outside of thechamber 200, whereby the heating element 412 can be controlledexternally. FIG. 4B illustrates a perspective view of the crucible 404,which has a dish shape in this example, with the diameter D as large aspossible within the dedicated zone in the chamber 200. The horizontalcross-sectional shape of the crucible 404 can be a square, a rectangle,an oval, a hexagon or any other shape. FIG. 4C illustrates a perspectiveview of the heating element 412, which is a spiral-shaped tungstenfilament and tightly surrounds the dish-shaped crucible 404, in thisexample. The heating element 412 can be formed in a mesh shape, ameander shape, a zig-zag shape or any other shape, as long as it isconfigured to heat the crucible 404 uniformly to control the evaporationrate of the source material effectively. In the present example, thediameter of the spiral is configured to be approximately the same as thediameter D of the dish-shaped crucible 404 of FIG. 4B.

Physical vapor deposition is an example of fabrication technique used insemiconductors, microelectronics and optical industries. The sourcematerial is typically heated and vaporized until its vapor pressure ishigh enough to produce a flux. The deposition onto the substrateinvolves purely physical process such as high-temperature vacuumevaporation with subsequent condensation or plasma sputter bombardment.Thus, line-of-sight transfer is typical for most of physical vapordeposition techniques, in which the direction of the vapor flux of thesource material is directed toward the substrate. Since particles tendto follow a straight path, films deposited by physical vapor depositionare generally directional, rather than conformal. In contrast, inchemical vapor deposition, chemical reaction takes place on thesubstrate surface to produce the conformal uniform morphology.

In view of the conventional chemical and physical vapor depositiontechniques, the fabrication technique based on the present system andmethod may be regarded as a hybrid of both techniques. Supposing thepresent system in FIG. 2 is configured to have one AX source in theevaporation unit 216 coupled to the side section of the chamber 200 andone BX₂ source in the evaporation unit 220 coupled to the bottomsection, the deposition mechanism can be qualitatively described asfollows: (i) the AX vapor is circulated to fill the chamber 200, whichis enabled by adjusting the evaporation rate of the AX compound via thetemperature control of the evaporation unit 216 and the flow speed ofthe AX vapor via the pressure control by using the pump unit 204,wherein both the evaporation unit 216 for AX and the pump unit 204 arecoupled to the side section of the chamber 200 to allow for effectivecontrol of the AX vapor flow; (ii) the BX₂ compound is heated in theevaporation unit 220 coupled to the bottom section, and the resultantBX₂ vapor travels substantially vertically directing to the substratestage 208, whereby the deposition of the BX₂ vapor is substantiallydirectional, following the vertical line-of-sight transfer, while thedeposition of the AX vapor is substantially less directional; (iii) theBX₂ vapor hits the substrate surface and is deposited thereoneffectively based partially on the good sticking coefficient andwettability of the BX₂ material; and (iv) the chemical reaction takesplace between the deposited BX₂ and the AX vapor existent in theproximity of the substrate surface. Thus, the present configurationeffectuates a substantially uniform chemical reaction on a large area ofthe substrate surface or multiple substrate surfaces, resulting in alarge-scale, uniform perovskite film with high crystallinity evenwithout annealing. Furthermore, an efficient chemical reaction on thesubstrate surface can be promoted, and thus the speed of the film growthcan be made significantly fast, by optimizing the evaporation rates ofsource materials via the respective temperature controls, and thecirculation of the AX vapor, hence the AX incorporation ratio to thedeposited BX₂, via the pressure control by using the pump unit 204.Furthermore, the initial deposition control and the film thicknesscontrol are enabled primarily by the open/close of the shutter 228 forthe substrate stage 208.

Therefore, the present fabrication process is inherently different froma typical physical vapor co-deposition process. In a typical physicalvapor co-deposition process, two evaporators need to be situatedside-by-side with an angle so that both vapor flows are directed at thesubstrate surface to have line-of-sight transfer of both sourcematerials. Accordingly, each of the vapor flows reaches the substratesurface at an angle, limiting the overlap region of the two vapor flows.That is, the stoichiometry of the resultant perovskite film in thecentral region is different from that in the edge region of the film.Therefore, the substrate size is limited, and the crystallinity of theresultant perovskite film tends to be of low quality even afterannealing because of non-uniform concentration of the two sourcematerials (e.g., AX and BX₂). Furthermore, the present system includesthe horizontally situated pump unit 204 to effectuate optimal flow ofthe AX vapor over the substrate surface, thereby optimizing the AXincorporation ratio to the deposited BX₂, whereas such flow control doesnot lead to benefits for physical vapor deposition because it isirrelevant to the operation principles based on a purely physicalprocess in the molecular regime.

FIG. 5 illustrates another example of the system configuration forfabricating a perovskite film according to an embodiment. The systemincludes a chamber 500 coupled to necessary parts. Similar to the systemconfiguration illustrated in FIG. 2, a pump unit 504 is coupled to theside section of the chamber 500 for generating near vacuum in thechamber 500; a substrate stage 508 is coupled to the top section of thechamber 500 for a substrate to be staged facing downward; a shutter 528is provided below the substrate stage 508 and is attached with a rod532, which is used to move the shutter 528 to expose or cover thesubstrate stage 508; the substrate stage 508 is coupled to a motor 512,which drives to rotate the substrate stage 508 around the central axis.In contrast to FIG. 2, in which a first evaporation unit 216 configuredfor generating the AX vapor is coupled to the side section of thechamber 200, in the system illustrated in FIG. 5 a first evaporationunit 516 is coupled to the bottom section of the chamber 500. Examplesof types of the first evaporation unit 516 coupled to the bottom sectioninclude a cell evaporator illustrated in FIG. 3A, an ampule with aheating element illustrated in FIG. 3B, and a crucible with a heatingelement as illustrated in FIGS. 4A-4C. A set of second evaporation units520, 521 and 522 are coupled also to the bottom section, wherein atleast one of the second evaporation units 520, 521 and 522 is configuredto generate the BX₂ vapor. Although not shown in FIG. 5, an evaporationshutter, such as the one 236 in the system illustrated in FIG. 2, may beprovided for each of the four evaporation units 516, 520, 521 and 522,for use for quick switching on/off of the vapor coming from theevaporation unit into active deposition. Monitors 540, 544, 545 and 546are provided above the evaporation units 516, 520, 521 and 522,respectively, to monitor the evaporation rates of the source materialscoming out from the respective evaporation units. A filter 550 may bedetachably attached at the opening of the pump 504. The filter 550 maybe made of a fine fabric mesh held by two concentric ring-shaped frames,as illustrated in FIG. 5.

A shield 524 is structured to separate the four evaporation units 516,520, 521 and 522 coupled to the bottom section of the chamber 500,defining individual zones corresponding to these evaporation units. Inthis example, the shield 524 includes four rectangular plates definingfour zones for the four evaporation units 516, 520, 521 and 522,respectively, to reduce thermal interference between different types ofvapors, enabling independent evaporation control of different materialsin the respective evaporation units. The structure of the shield 524 isconfigured to include vertically upright plates to define the individualzones with a predetermined height in the chamber 500 for facilitatingindependent deposition behaviors. The shield structure around the firstevaporation unit 516, however, is configured differently from the shieldstructure around each of the second evaporation units 520, 521 and 522,in that a top shield portion 525 is provided above the first evaporationunit 516. That is, the shield structure 524 includes the top shieldportion 525 only above the first evaporation unit 516. The shieldstructure around the first evaporation unit 516 allows for the AX vaporto effectively circulate in the chamber 500, without directly hittingthe substrate surface. On the other hand, the shield structure aroundeach of the second evaporation unit 520, 521 and 522 allows for thedeposition of the source vapor therefrom to be substantiallydirectional, following the line-of-sight transfer to the substratesurface. The zone designated for each of the evaporation units in thesecond set, 520, 521 and 522, is configured to have a horizontalcross-sectional area that is open and facing the substrate surface. Thehorizontal cross-sectional areas of the separate zones designated forthe evaporation units in the second set, 520, 521 and 522, together withthe rotation speed of the substrate stage 508, can be predeterminedbased on desired deposition rates and growth composition. Instead of aplate structure, a chimney-like structure having a shape such as acylinder, a funnel, etc. may be configured to shield each evaporationunit from the others. In this case, the opening portion of the shieldstructure for the first evaporation unit 516 may be oriented to faceaway from the substrate stage 508 so as to reduce the direct depositionof the AX vapor onto the substrate surface and to promote thecirculation in the chamber 500. Additionally or alternatively to havingthe top shield 525 above the first evaporator unit 516 or configuringthe opening portion of the shield structure for the first evaporationunit 516 to face away from the substrate surface, the first evaporationunit 516 itself may be placed horizontally away from the secondevaporation units 520, 521 and 522, so that the overlap between thehorizontal cross-sectional area of the substrate stage 508 and that ofthe first evaporation unit 516 is avoided.

In the system illustrated in FIG. 5, one first evaporation unit 516 andthree second evaporation units 520, 521 and 522 are coupled to thebottom section of the chamber 500. However, one or more firstevaporation units may be coupled to the bottom section of the chamber500. A shield is structured around each of the evaporation unit in thefirst set so as to reduce direct deposition of the source vapor onto thesubstrate surface, thereby promoting circulation of the source vapor inthe chamber 500. Alternatively, all the first evaporation units may becoupled to the side section of the chamber, as illustrated in FIG. 2.Yet alternatively, one or more of the first evaporation units may becoupled to the bottom section, and the other first evaporation units maybe coupled to the side section. That is, the present system isconfigured to include a first set of evaporation units comprising one ormore evaporation units, each coupled to the side section of the chamberor to the bottom section of the chamber with a proper shield structurearound it, wherein at least one evaporation unit in the first set isconfigured to generate the AX vapor.

The present system illustrated in FIG. 2 or FIG. 5 is configured toinclude multiple sources and is capable of controlling the growthprofile at the monolayer or sub-monolayer level based primarily on theevaporation rates, the rotation speed of the substrate stage 208/508 andthe zoning afforded by the shield 224/524. The operation principle ofthe present system and method including multiple sources is explainedbelow.

Here, the case of having one AX source and N BX₂ sources is consideredfor illustrative purposes. The AX source is contained in the evaporationunit in the first set that is coupled to the side section of the chamberor to the bottom section of the chamber with a proper shield structurearound it, as illustrated in FIG. 2 or 5, for generating the AX vaporwith the evaporation rate of R_(AX). The N BX₂ sources are contained inthe S₁, S₂ . . . and S_(N) evaporation units in the second set coupledto the bottom section of the chamber, for generating the S₁, S₂ . . .and S_(N) vapors with the evaporation rates of R₁, R₂ . . . and R_(N),respectively. The rotation speed and evaporation temperatures arepredetermined to provide desired evaporation rates, R_(AX), R₁, R₂ . . .and R_(N) in the unit of ML/rotation, where ML stands for monolayer. Therotation speed is set to be M Hz. For the evaporation rates, the unit ofML/sec can alternatively be used, where ML/sec=ML/(M rotations) holdsfor the case of using the rotation speed of M Hz. Each of the S₁, S₂ . .. and S_(N) evaporation units is placed in a zone defined by the shield,as illustrated in FIG. 2 or 5, and the horizontal cross-sectional areasof the zones designated for these evaporation units are A₁, A₂ . . . andA_(N), respectively. Assuming R_(AX) is properly chosen to ensurestoichiometry and all evaporation shutters are open during deposition,after each rotation, i.e., the rotation of the substrate stage 208/508around its central axis by 360°, the deposited film has the followingstructure:

$\begin{matrix}{{{Sub}\text{-}{Layer}\mspace{14mu} 1\mspace{14mu} {comprises}\mspace{14mu} S_{1}\mspace{14mu} {with}\mspace{14mu} {thickness}\mspace{14mu} t_{1}} = {R_{1}^{\prime}\left\lbrack {A_{1}/\left( {A_{1} + A_{2} + \ldots + A_{N}} \right)} \right\rbrack}} & {{Eq}.\mspace{14mu} (1)} \\{{{Sub}\text{-}{Layer}\mspace{14mu} 2\mspace{14mu} {comprises}\mspace{14mu} S_{2}\mspace{14mu} {with}\mspace{14mu} {thickness}\mspace{14mu} t_{2}} = {R_{2}^{\prime}\left\lbrack {A_{2}/\left( {A_{1} + A_{2} + \ldots + A_{N}} \right)} \right\rbrack}} & \; \\{\mspace{79mu} \ldots} & \; \\{{{Sub}\text{-}{Layer}\mspace{14mu} N\mspace{11mu} {comprises}\mspace{14mu} S_{N}\mspace{14mu} {with}\mspace{14mu} {thickness}\mspace{14mu} t_{N}} = {R_{N}^{\prime}\left\lbrack {A_{N}/\left( {A_{1} + A_{2} + \ldots + A_{N}} \right)} \right\rbrack}} & \;\end{matrix}$

The above multilayer structure, comprising Sub-Layer 1 through Sub-LayerN, formed after one rotation can be defined as a unit layer (t₁/t₂/ . .. /t_(N)). After one second, the film comprises M repeating unit layersas follows:

(t ₁ /t ₂ / . . . /t _(N))/(t ₁ /t ₂ / . . . /t _(N))/ . . . /(t ₁ /t ₂/ . . . /t _(N)).  Eq. (2)

Strictly speaking, in the above illustration, the initial layers will bedifferent from location to location of the substrate surface if onelarge substrate is used, since the initial vapors of different BX₂materials hit different locations of the substrate surface. Thereafter,the deposition will proceed like forming a swirl of each depositedmaterial as the substrate stage 208/508 rotates many times. However, inactual fabrications, the deposition continues to grow a film with athickness of 50-400 nm, for example, containing hundreds or thousands oflayers. Thus, the difference in initial layers becomes non-essential,and the growth profile can be approximated by layer-by-layer inpractice. If multiple substrates are used, these substrates mounted ondifferent locations on the substrate stage 208/508 have differentinitial layers. Again, the difference in initial layers becomesnon-essential after the deposition of hundreds or thousands layers. Forexample, supposing Structure 1=S₁/S₂/S₁/S₂ . . . is formed on onesubstrate, and Structure 2=S₂/S₁/S₂/S₁ . . . is formed on anothersubstrate, these structures can be viewed as: Structure 2=S₂/S₁/S₂/S₁ .. . =S₂/(Structure 1). Therefore, in the practical application of thepresent system and method, the difference in the initial layers becomesnon-essential. That is, similar to using one large substrate, two ormore separate substrates can be placed radially on the substrates stage208 to achieve the layer-by-layer growth. For example, a round substratestage with a diameter of ˜9 cm and four square substrates with each sideof ˜1.5 cm may be used, wherein the four substrates are radially placedon the substrate stage ˜90° apart from each of the adjacent ones.

Examples of fabricating perovskite films using the present multi-sourcesystem and method are described below, wherein the numerical values aregiven only for illustrative purposes.

Example 1

There are two evaporation units, containing two BX₂ sources, coupled tothe bottom section of the chamber and placed in two zones, respectively,where the two BX₂ compounds are S₁═SnI₂ and S₂═PbI₂; there is oneevaporation unit containing AX=MAI, which is coupled to the side sectionof the chamber or to the bottom section of the chamber with a propershield structure around it, as illustrated in FIG. 2 or 5. Theevaporation rates are: R₁=1 ML/rotation, R₂=1 ML/rotation, andR_(MAI)=0.5 ML/rotation. The zone area distribution is: A₁=A₂=0.5′(A₁+A₂). The rotation speed is 2 Hz. The evaporation shutters for allthe sources are open during deposition. After each rotation, thedeposited film comprises the Pb—Sn mixed perovskite with stoichiometrybalanced at 1:1 ratio: 0.5 ML MASnI₃/0.5 ML MAPbI₃. After one second,the film comprises two repeating unit layers, i.e., (0.5 ML MASnI₃/0.5ML MAPbI₃)/(0.5 ML MASnI₃/0.5 ML MAPbI₃).

Example 2

There are two evaporation units, containing two BX₂ sources, coupled tothe bottom section of the chamber and placed in two zones, respectively,where the two BX₂ compounds are S₁═SnI₂ and S₂═SnBr₂; there is oneevaporation unit containing AX=MAI, which is coupled to the side sectionof the chamber or to the bottom section of the chamber with a propershield structure around it, as illustrated in FIG. 2 or 5. Theevaporation rates are: R₁=4 ML/rotation, R₂=1 ML/rotation, andR_(MAI)=2.5 ML/rotation. The zone area distribution is: A₁=A₂=0.5′(A₁+A₂). The rotation speed is 3 Hz. The evaporation shutters for allthe sources are open during deposition. After each rotation, i.e., therotation of the substrate stage 208/508 around its central axis by 360°,the deposited film comprises the I—Br mixed perovskite withstoichiometry balanced at 13:2 ratio: 2 ML MASnI₃/0.5 ML MASnIBr₂. Afterone second, the film comprises 3 repeating unit layers, i.e., (2 MLMASnI₃/0.5 ML MASnIBr₂)/(2 ML MASnI₃/0.5 ML MASnIBr₂)/(2 ML MASnI₃/0.5ML MASnIBr₂).

Example 3

There are three evaporation units, containing three source materials,coupled to the bottom section of the chamber and placed in three zones,respectively, where the two sources are BX₂ compounds, S₁═SnI₂ andS₂═SnBr₂, and the third source is a non-volatile dopant source,S₃═Bi(NO₃)₃ (or BiX₃ where X═F, Cl, Br or I); there is one evaporationunit containing AX=FAI, which is coupled to the side section of thechamber or to the bottom section of the chamber with a proper shieldstructure around it, as illustrated in FIG. 2 or 5. The evaporationrates are: R₁=2.7 ML/rotation, R₂=2.7 ML/rotation, R₃=0.6 ML/rotation,and R_(FAI)=0.67 ML/rotation. The zone area distribution is:A₁=A₂=A₃=0.33′ (A₁+A₂+A₃). The rotation speed is 5 Hz. The evaporationshutters for all the sources are open during deposition. After eachrotation, the deposited film comprises the I—Br mixed formamidiniumperovskite with a 10% doping concentration of Bi with stoichiometrybalanced at 2:1 ratio: 1 ML FASn_(0.9)Bi_(0.1)I₃/1 MLFASn_(0.9)Bi_(0.1)IBr₂. After one second, the film comprises threerepeating unit layers, i.e., (1 ML FASn_(0.9)Bi_(0.1)I₃/1 MLFASn_(0.9)Bi_(0.1)IBr₂)/(1 ML FASn_(0.9)Bi_(0.1)I₃/1 MLFASn_(0.9)Bi_(0.1)IBr₂)/(1 ML FASn_(0.9)Bi_(0.1)I₃/1 MLFASn_(0.9)Bi_(0.1)IBr₂).

In the above Example 3, the doping concentration of 10% is considered asan example. Generally, it is difficult to achieve PPM-level dopingconcentrations. However, for solar cell applications, the requireddoping concentration is expected to be higher for polycrystalline filmsincluding perovskite films than monocrystalline films such as singlecrystal Si films. It is not uncommon to find reported dopingconcentrations as high as 10% in organic amorphous semiconductors. Inview of the tolerance levels of various growth parameters, it isexpected that the present system and method can be used to realize adoping concentration of 0.1% or smaller.

In the above Example 3, one of the evaporation units coupled to thebottom section is used to generate vapor of a non-volatile dopantsource. If the dopant material is volatile, it is preferable to use oneof the evaporation units coupled to the side section of the chamber orto the bottom section of the chamber with a proper shield structurearound it, as illustrated in FIG. 2 or 5. This is because a volatiledopant source tends to readily vaporize at a relatively low temperature,and thus the volatile dopant source is qualitatively similar incharacteristics to the organic AX compounds (MACl, MABr, MAI, FACl,FABr, FAI and the like). The characteristics can be utilized in thepresent system and method to circulate the vapor in the chamber foroptimal incorporation of the volatile material to the deposited BX₂.

Example 4

There are two evaporation units, containing two source materials,coupled to the bottom section of the chamber and placed in two zones,respectively, where the two sources are BX₂ compounds, S₁═SnI₂ andS₂═SnBr₂; there are two evaporation units, each coupled to the sidesection of the chamber or to the bottom section of the chamber with aproper shield structure around it, as illustrated in FIG. 2 or 5, onecontaining AX=FAI and the other containing a volatile dopant materialI₂. The evaporation unit including an ampule with an evaporation shutterin the form of a valve, such as illustrated in FIG. 3B, may be used forgenerating vapor of the volatile dopant material I₂.

Example 5

There are two evaporation units, containing two source materials,coupled to the bottom section of the chamber and placed in two zones,respectively, where the two sources are BX₂ compounds, S₁═PbI₂ andS₂═SnI₂; there is one evaporation unit containing AX=MAI, which iscoupled to the side section of the chamber or to the bottom section ofthe chamber with a proper shield structure around it, as illustrated inFIG. 2 or 5. The evaporation rates are: R₁=1 ML/rotation, R₂=1ML/rotation, and R_(MAI)=0.5 ML/rotation. The zone area distribution is:A₁=A₂=0.5′ (A₁+A₂). The rotation speed is 1 Hz. A two-step depositionprocess is carried out by having the evaporation shutter open forS₁═PbI₂ and the evaporation shutter closed for S₂═SnI₂ in the firststep, and vice versa in the second step. After the first step with 600rotations=600 seconds, the deposited film comprises 600′ (0.5 MLMAPbI₃)=300 ML MAPbI₃. After the second step with 600 rotations=600seconds, the deposited film comprises 300′ (0.5 ML MASnI₃)=300 ML MASnI₃on top of the first deposited film, resulting in a perovskiteheterostructure having 300 ML MAPbI₃/300 ML MASnI₃. Thus, the perovskiteheterostructure can be obtained by using the two- or more-stepdeposition process based on the present system and method.

According to the present system and method, the layer thickness andcomposition can be controlled at the monolayer or sub-monolayer levelvia the rotation speed of the substrate stage 208/508 and the zone areadistribution as expressed in Eq. (1), in addition to the evaporationtemperatures that determine the respective evaporation rates.Accordingly, as exemplified in the above Examples 1-5, the presentsystem and method can be configured to fabricate various perovskitestructures suitable for a wide variety of advanced applications. Thepresent system includes: a chamber having a closed hollow structure thathas a side section along a vertical direction and top and bottomsections along a horizontal direction; a substrate stage coupled to thetop section of the chamber and configured to have a stage surface facingvertically downward for a substrate to be placed and to rotate around acentral axis at a rotation speed; a first set of evaporation unitscomprising one or more evaporation units, each coupled to the sidesection or to the bottom section of the chamber, for generating vaporsof one or more first source materials with one or more first evaporationrates, respectively, wherein at least one of the one or more firstsource materials is the AX; a second set of evaporation units comprisingtwo or more evaporation units coupled to the bottom section of thechamber for generating vapors of two or more second source materialswith two or more second evaporation rates, respectively, wherein atleast one of the two or more second source materials is the BX₂; and ashield for separating two or more evaporation units coupled to thebottom section of the chamber, defining two or more zones designated forthe two or more evaporation units coupled to the bottom section,respectively. The zone designated for each of the two or moreevaporation units in the second set is configured to have a horizontalcross-sectional area that is open and facing the substrate surface. Thefirst set of evaporation units, the second set of evaporation units andthe shield are configured to enable deposition of each of the two ormore second source materials to be substantially directional by verticalline-of-sight transfer, and deposition of each of the one or more firstsource materials to be substantially less directional by circulating thevapor of the first source material in the chamber. At least the one ormore first evaporation rates, the two or more second evaporation rates,the rotation speed and the two or more horizontal cross-sectional areasare adjusted to control composition and thickness of a unit layer tofabricate the perovskite film comprising a plurality of unit layers,wherein each unit layer is formed by one rotation of the substratestage.

The present system and method can be configured to fabricate dopedperovskite films, as in Examples 3 and 4 above. It is reported thatundoped (or unintentionally doped) perovskites are mostly n-type. Thus,successful synthesis of p-type perovskites can lead to the p-n junctionformation for a perovskite solar cell. The p-type perovskite may begenerally expressed as D p-doped ABX₃, where D is a dopant material suchas the volatile I₂ or the non-volatile Bi(NO₃)₃, BiX₃, and the like.These dopants provide free holes when incorporated in the ABX₃structure. To intentionally n-dope a perovskite film, n-type dopantmaterials E can be used, where E=methylammonium, formamidinium, Pb,InX₃, and the like. These dopants provide free electrons whenincorporated in the ABX₃ structure to give E n-doped ABX₃. In addition,the present system and method enables dopant profile tuning by graduallyadjusting the rotation speed and/or the zone area distribution, wherebythe doping concentration can be gradually varied in the perovskite film.

Furthermore, the present system and method can be configured tofabricate perovskite heterostructures, as in Example 5, for formingTandem or multi-junction structures. Examples of such structuresinclude: a perovskite p-n junction, i.e., p-type perovskite/n-typeperovskite or vice versa; a perovskite film on Si, such as p-typeSi/n-type perovskite, n-type Si/p-type perovskite, etc.; a perovskitefilm on an organic material, such as PCBM/perovskite, where PCBM standsfor Phenyl-C61-Butyric acid Methyl ester. Tandem and multi-junctionsolar cells generally exhibit high conversion efficiency. It has beenreported that a prior art example of a general Tandem solar cell canexhibit 47% efficiency under the AM1.5G spectrum, having bandgaps of1.63 eV and 0.96 eV. It is expected that comparable or even higherefficiency can be achieved by using a Tandem or multi-junctionperovskite solar cell.

Additionally, the present system and method can be configured forbandgap engineering. For example, the bandgap of a mixed-halideperovskite such as FAPbI_(x)Br_(3-x) can be tuned within a certain rangeby varying the halide composition, enabling variation in color, forexample. Other examples include MASnxPb_(1-x)I₃, in which the bandgapcan be varied by varying the metal composition. The present system andmethod, via implementation of multiple source and independent control ofa wide variety of growth parameters, offers suitable means for bandgapengineering, such as in situ tuning, precise thickness control,concentration profile design, and so on.

Furthermore, the present system and method can be configured to create aone-station process for the full cell fabrication by implementingdeposition of the electron or hole collection materials, top electrodematerial, encapsulation material and other materials all in the samechamber.

Referring back to the perovskite structure illustrated in FIG. 1, it isgenerally known that materials can be considered as candidates for solarcells as long as the tolerance factor, t, defined below does not deviatesignificantly from 1:

$\begin{matrix}{{t = \frac{r_{A} + r_{X}}{\sqrt{2}\left( {r_{B} + r_{X}} \right)}},} & {{Eq}.\mspace{14mu} (3)}\end{matrix}$

where r_(A) is the radius of the A-cation, r_(B) is the radius of theB-cation, and r_(X) is the radius of the X-anion. The above tolerancefactor, t, known to those skilled in the art as the Goldschmidttolerance factor, is an indicator for the stability and distortion ofcrystal structures. Thus, based on the design guideline using thetolerance factor, new perovskite materials for solar cells can bedevised by using the present system and method that implements multiplesources and independent control of a wide variety of growth parametersin a versatile and comprehensive fashion.

FIG. 6 is a flowchart illustrating an example of the fabrication processof a perovskite film using the present system. This flowchart describesa general case of having S1 ₁, S1 ₂ . . . and S1 _(M) source materialsprovided in the first set of evaporation units, respectively, eachcoupled to the side section of the chamber as illustrated in FIG. 2 orto the bottom section of the chamber with a proper shield structurearound it, as illustrated in FIG. 5, wherein M≧1 and at least one of theS1 ₁, S1 ₂ . . . and S1 _(M) source materials is the AX compound. Inthis general case, S2 ₁, S2 ₂ . . . and S2 _(N) source materials areprovided in the second set of evaporation units, respectively, coupledto the bottom section of the chamber, wherein N≧2 and at least one ofthe S2 ₁, S2 ₂ . . . and S2 _(N) source materials is the BX₂ compound.The shield is provided to define zones respectively designated for theevaporation units coupled to the bottom section of the chamber. Thezones for the second set of evaporation units, containing S2 ₁, S2 ₂ . .. and S2 _(N) source materials, have horizontal cross-sectional areasA₁, A₂ . . . and A_(N), respectively, which are open and facing thesubstrate stage 208/508. As explained earlier, a top shield portion,such as 525 in FIG. 5, may be provided above each of the evaporationunits in the first set coupled to the bottom section of the chamber.Alternatively, a chimney-like structure having a shape such as acylinder, a funnel, etc. may be configured to shield each evaporationunit from the others coupled to the bottom section of the chamber. Inthis case, the opening portion of the shield structure for each of theevaporation units in the first set may be oriented to face away from thesubstrate stage. Additionally or alternatively, each of the evaporationunits in the first set may be placed horizontally away from theevaporation units in the second set, so as to avoid the overlap betweenthe horizontal cross-sectional area of the substrate stage and that ofeach of the evaporation units in the first set coupled to the bottomsection. Yet alternatively, all the evaporation units in the first setmay be coupled to the side section of the chamber, as illustrated inFIG. 2. One or more substrates, collectively called a substrate in thisdocument, are placed on the substrate stage 208/508 facing downward.Examples of substrate materials include: fluorine-dope tin oxide (FTO)glass with electron collection layer (ECL) such as TiO₂ compact layer orZnO thin film; indium tin oxide (ITO) glass with hole collection layer(HCL) such as PEDOT:PSS and NiO, where PEDOT:PSS stands forpoly(3,4-ethylenedioxythiophene) polystyrene sulfonate; flexiblesubstrate, such as polyethylene terephthalate, with either ECL or HCL.

The inside of the chamber 200/500 is pumped to a predetermined vacuumlevel by using the pump unit 204/504. The substrate temperature, theevaporation temperatures for heating the source materials in theevaporation units to provide intended evaporation rates, respectively,the rotation speed of the substrate stage 208/508, the zone areadistribution A₁, A₂ . . . and A_(N), the intended film thickness, andother growth parameters can be predetermined before the depositionstarts and/or can be adjusted during the deposition. The evaporationshutters, such as 236 in FIG. 2, may be provided respectively for theevaporation units in the second set and can be adjusted before and/orduring the multi-source deposition. Each evaporation shutter can beopened/closed for quick switching on/off of the vertical transfer of thevapor during the deposition. Similarly, the evaporation shutter, such as320 in FIG. 3A or 340 in FIG. 3B, provided for the evaporation unit inthe first set can be adjusted before and/or during the multi-sourcedeposition.

In step 604 of FIG. 6, the temperature of the substrate stage 208/508 iscontrolled to provide the predetermined substrate temperature. Thetemperature of the substrate stage 208/508 can be controlled to provideuniform cooling or heating to the substrate, ranging from −10° C. up to350° C., for example. In step 608, the shutter 228/528, which isprovided just below the substrate stage 208/508, is closed to cover thesubstrate. The rotation of the substrate stage 208/508 can be started.In step 612, the evaporation rates of the source materials are adjustedby controlling the evaporation temperatures in the evaporation units,respectively. For example, the crucible 404 in FIG. 4A can be heated bythe heating element 412 of the evaporation unit 220 in the second set tothe temperature that generates the BX₂ vapor at the predeterminedevaporation rate; the crucible 308 in FIG. 3A can be heated by theheating element 316 of the evaporation unit 216 in the first set to thetemperature that generates the AX vapor at the predetermined evaporationrate. When a cell evaporator such as the example in FIG. 3A is used forthe AX source, it is preferable, for enhancing the uniformity of theresultant perovskite film, that the evaporation shutter 320 is adjustedto be at a position, which allows for the AX vapor to output from theevaporator efficiently and yet prevents the AX vapor from directlyhitting the substrate during deposition. The pump unit 204/504 iscoupled to the side section of the chamber 200/500 so as to have thepumping direction substantially along the horizontal direction. Thisconfiguration allows for the flow of the AX vapor to circulate in thechamber 200/500 effectively so as to facilitate the chemical-reactiontype of deposition on the substrate. In step 616, the shutter 228/528,which is provided just below the substrate stage 208/508, is opened tostart the deposition. In step 620, the thickness of the perovskite filmgrowing on the substrate is monitored in situ by using at least one ofthe monitors provided near the evaporation units, respectively. Thedeposition rate and the film thickness in situ can be estimated based onthe monitored evaporation rate using the tooling factor calculation, forexample. In this calculation, the ratio between the measured filmthickness and the indicated film thickness (as indicated by themonitored evaporation rate) is obtained during a trial run; thereafter,the ratio can be used to obtain the in situ film thickness by factoringin the evaporation rate during deposition as observed by the monitor. Instep 624, when the film thickness reaches the predetermined thickness,the shutter 228/528 is closed to interrupt the deposition.

The present system and method can be configured to form a wetting layeron the substrate to enhance the quality of the subsequent perovskitefilm in terms of uniformity and wettability. The wetting layer may becomprised of a metal halide, BX₂, where B═Pb or Sn and X═Cl, Br or I,which can be provided in one of the second set of the evaporation unitscoupled to the bottom section of the chamber 200/500. PbI₂ or PbCl₂ maybe preferably chosen for the wetting layer material. The fabricationprocess described in the flowchart of FIG. 6 can be modified toaccommodate the optional growth of the wetting layer prior to theperovskite film growth. FIG. 7 is a flowchart illustrating an example ofa modified process to include the growth of the wetting layer comprisingthe BX₂ prior to the perovskite film growth using the present system.Initially, in step 704, the evaporation shutter for the evaporation unitcontaining the source material BX₂ for the wetting layer is open,whereas the other shutters for the evaporation units containing sourcematerials not needed for forming the wetting layer are closed. This stepmay be carried out before step 604, between steps 604 and 608, or afterstep 608 that is the case illustrated in FIG. 7. After the shutter228/528 is closed and the rotation of the substrate stage 208/508 isstarted, the evaporation rate of the BX₂ in one of the second set ofevaporation units is adjusted by controlling the evaporation temperaturein step 708. In step 712, the shutter 228/528, which is provided justbelow the substrate stage 208/508, is opened to start the deposition ofthe BX₂ vapor. In step 716, the thickness of the wetting layer growingon the substrate is monitored in situ by using the monitor provided nearthe evaporation unit containing the BX₂ source material. The toolingfactor obtained by the tooling factor calculation may be used toestimate the thickness based on the monitored evaporation rate. In step720, when the wetting layer thickness reaches the predeterminedthickness, the shutter 228/528 is closed to interrupt the deposition ofthe BX₂ vapor. For example, a wetting layer with a thickness of about 50nm may be formed prior to forming a subsequent perovskite film with athickness of 200 nm. The deposition of the perovskite film on top of thewetting layer may follow by carrying out step 604 and the subsequentsteps.

The present system and method can be configured to fabricate perovskiteheterostructures having multiple sub-films, as in Example 5, for forminga Tandem or multi-junction structure or for bandgap engineering.One-station process can be configured by preparing all the sourcematerials needed for forming different sub-films, and adjusting theevaporation rates of the needed source materials for each sub-filmgrowth by controlling the corresponding evaporation temperatures whileclosing the evaporation shutters provided for the evaporation unitscontaining source materials not needed for forming the particularsub-film. FIG. 8 is a flowchart illustrating an example of thefabrication process of a perovskite heterostructure using the presentsystem. Two or more different perovskite sub-films can be fabricatedsequentially by repeating steps 804-828 in FIG. 8. In step 804 of FIG.8, the temperature of the substrate stage 208/508 is controlled toprovide the predetermined substrate temperature. In step 808, theshutter 228/528, which is provided just below the substrate stage208/508, is closed to cover the substrate. The rotation of the substratestage 208/508 can be started. For forming the N-th sub-film having theN-th type of perovskite material, the evaporation shutters for theevaporation units containing the source materials needed for the N-thsub-film growth are opened, while the other evaporation shutters areclosed in step 812. This step may be carried out before step 804,between steps 804 and 808, or after step 808 that is the caseillustrated in FIG. 8. In step 816, the evaporation rates of the sourcematerials needed for forming the N-th sub-film are adjusted bycontrolling the respective evaporation temperatures associated with theevaporation units. In step 820, the shutter 228/528, which is providedjust below the substrate stage 208/508, is opened to start thedeposition. In step 824, the thickness of the N-th sub-film growing onthe previous N−1 sub-film or on the substrate is monitored in situ byusing at least one of the monitors provided respectively near theevaporation units generating the vapor sources needed for forming theN-th sub-film. In step 828, when the N-th sub-film thickness reaches thepredetermined thickness, the shutter 228/528 is closed to interrupt thedeposition.

In any of the fabrication processes based on the present system andmethod, such as those exemplified in FIG. 6-8, the order of some of thesteps may be changed, combined or separated for ease and convenience ofcarrying out the process. For example, in FIG. 6, controlling thetemperature of the substrate stage in step 604 and closing the shutter228/528 in step 608 may be reversed in order. In another example,starting the rotation of the substrate stage 208/508 may be delayeduntil just prior to opening the shutter 228/528. Additionally, each ofthe evaporation shutters for the evaporation units may be opened, closedor adjusted to a certain position before and/or during the depositiondepending on the in situ conditions to optimize the growth quality.

FIG. 9 is a plot of the J-V curve representing the photovoltaic devicecharacterization of a solar cell including the chloride iodideperovskite film, CH₃NH₃PbI_(3-X)Cl_(X) film, grown by using the presentfabrication system and method as illustrated in FIG. 2. The systemincluded two evaporation units coupled to the bottom section of thechamber 200, in which the PbI₂ and PbCl₂ compounds were provided togenerate respective vapor sources in two zones defined by the shield224; one evaporation unit coupled to the side section of the chamber200, in which the MAI compound was provided to generate its vapor. TheTiO₂/FTO substrate was used for the present deposition. The perovskitefilm was grown up to a thickness of about 200 nm on top of the PbI₂wetting layer with a thickness of about 50 nm. The measurements of theJ-V curve were carried out under a simulated AM1.5G solar irradiation of100 mW/cm². This plot shows that the short circuit current density (Jsc)is 15.9 mA/cm², the open circuit voltage (Voc) is 1.04V, and the fillfactor (FF) is 0.582. This sample has the power conversion efficiency(PCE) of about 9.6%. All solar cells from the same batch depositionshowed the similar performance, thereby indicating a device yield of100%.

FIG. 10 is a plot of the measured absorbance of theCH₃NH₃PbI_(3-X)Cl_(X) film. The sharp rise observed at ˜780 nmcorresponds to a bandgap of 1.59 eV. The second onset at ˜500 nm also isconsidered to correspond to the intrinsic absorption of a perovskitematerial.

FIG. 11 is a photo showing the atomic force microscopy (AFM) images ofthe CH₃NH₃PbI_(3-X)Cl_(X) film. Images with scan sizes of 2×2 μm² and20×20 μm² are shown in (A) and (B), respectively. The AFM images showthat the typical root-mean square (RMS) roughness of the film is about37 nm, which is considered to be small, thereby indicating theuniformity of the perovskite film grown by the present fabricationsystem and method. Perovskite film thickness was determined to beapproximately 200 nm by measuring the edge between perovskite film andbare substrate covered by a shadow mask (i.e. a half of the substrate iscovered).

FIG. 12 is a plot showing the X-ray diffraction (XRD) spectrum of theCH₃NH₃PbI_(3-X)Cl_(X) film exhibiting the maximum PCE of 9.6% as shownin FIG. 9. This XRD spectrum shows the organometal halide perovskitecharacteristics having peaks at 14.04, 28.42 and 43.08 degreescorresponding to the (110), (220) and (330) planes of the orthorhombicstructure. It should be noted that the peak (110) is stronger than the(220) peak even in the absence of annealing in the present process. Thepeak labeled with PbI₂ is considered to originate from the PbI₂ wettinglayer.

FIG. 13 is a photo showing the AFM images of the structure of Si/100 nmperovskite/50 nm spiro-MeOTAD in (A) and the AFM image of the structureof Si/10 nm PbCl₂/100 nm perovskite/50 nm spiro-MeOTAD in (B), where theheight scales are the same in (A) and (B). These two images compare theeffect of adding a wetting layer prior to the perovskite film growth.PbCl₂ is used to form the wetting layer on the Si substrate in (B),whereas no wetting layer is included in the structure in (A).spiro-MeOTAD is an organic semiconductor and usually forms an amorphousfilm that is conformal to the film underneath. For example, whendeposited on a typical Si substrate that is very flat, the spiro-MeOTADfilm exhibits a roughness of about 0.1-0.2 nm. That is, the morphologyroughness of the perovskite film can be observed in the AFM image of thestructure capped with spiro-MeOTAD. Comparison between the AFM images in(A) and (B) reveals that significant improvement in film smoothness wasachieved by the addition of the PbCl₂ wetting layer of about 10 nm inthickness in this example. Subsequent experiments indicated that addingthe PbCl₂ wetting layer or the PbI₂ layer gives rise to similar effectsin terms of improving the film smoothness of the perovskite.

While this document contains many specifics, these should not beconstrued as limitations on the scope of an invention or of what may beclaimed, but rather as descriptions of features specific to particularembodiments of the invention. Certain features that are described inthis document in the context of separate embodiments can also beimplemented in combination in a single embodiment. Conversely, variousfeatures that are described in the context of a single embodiment canalso be implemented in multiple embodiments separately or in anysuitable subcombination. Moreover, although features may be describedabove as acting in certain combinations and even initially claimed assuch, one or more features from a claimed combination can in some casesbe exercised from the combination, and the claimed combination may bedirected to a subcombination or a variation of a subcombination.

1-34. (canceled)
 35. A system for fabricating a perovskite film, whereinsource materials include an organic halide compound AX and a metalhalide compound BX₂, wherein halogen X in the AX and halogen X in theBX₂ are the same or different, the system comprising: a chamber having aclosed hollow structure that has a side section along a verticaldirection and top and bottom sections along a horizontal direction; asubstrate stage coupled to the top section of the chamber and configuredto have a stage surface facing vertically downward for a substrate to beplaced and to rotate around a central axis at a rotation speed; a firstset of evaporation units comprising one or more evaporation units, eachcoupled to the side section or to the bottom section of the chamber, forgenerating vapors of one or more first source materials with one or morefirst evaporation rates, respectively, wherein at least one of the oneor more first source materials is the AX; a second set of evaporationunits comprising two or more evaporation units coupled to the bottomsection of the chamber for generating vapors of two or more secondsource materials with two or more second evaporation rates,respectively, wherein at least one of the two or more second sourcematerials is the BX₂; and a shield for separating two or moreevaporation units coupled to the bottom section of the chamber, definingtwo or more zones designated for the two or more evaporation unitscoupled to the bottom section, respectively, wherein the zone designatedfor each of the two or more evaporation units in the second set isconfigured to have a horizontal cross-sectional area that is open andfacing the substrate surface, wherein the first set of evaporationunits, the second set of evaporation units and the shield are configuredto enable deposition of each of the two or more second source materialsto be substantially directional by vertical line-of-sight transfer, anddeposition of each of the one or more first source materials to besubstantially less directional by circulating the vapor of the firstsource material in the chamber.
 36. The system of claim 35, wherein atleast the one or more first evaporation rates, the two or more secondevaporation rates, the rotation speed and the two or more horizontalcross-sectional areas are adjusted to control composition and thicknessof a unit layer to fabricate the perovskite film comprising a pluralityof unit layers, wherein each unit layer is formed by one rotation of thesubstrate stage.
 37. The system of claim 35, wherein at least one of theone or more evaporation units in the first set is coupled to the sidesection of the chamber.
 38. The system of claim 35, wherein at least oneof the one or more evaporation units in the first set is coupled to thebottom section of the chamber, and the shield is configured to include atop shield portion above the at least one of the one or more evaporationunits in the first set to promote the circulation of the vapor of thefirst source material therefrom by reducing the vapor directly hittingthe substrate.
 39. The system of claim 35, wherein at least one of theone or more evaporation units in the first set is coupled to the bottomsection of the chamber, and an opening portion of the shield for the atleast one of the one or more evaporation units in the first set isoriented to face away from the substrate surface to promote thecirculation of the vapor of the first source material therefrom byreducing the vapor directly hitting the substrate.
 40. The system ofclaim 35, wherein at least one of the one or more evaporation units inthe first set is coupled to the bottom section of the chamber away fromthe two or more evaporation units in the second set to avoid an overlapbetween a horizontal cross-sectional area of the substrate surface andthat of the at least one of the one or more evaporation units in thefirst set.
 41. The system of claim 36, wherein the two or more secondsource materials are two or more types of the BX₂, respectively, whereinat least the one or more first evaporation rates, the two or more secondevaporation rates, the rotation speed and the two or more horizontalcross-sectional areas are adjusted to form two or more sub-layers in theunit layer, the two or more sub-layers including at least elements ofthe two or more types of the BX₂, respectively.
 42. The system of claim36, wherein one of the two or more second source materials is a dopantmaterial, wherein at least the one or more first evaporation rates, thetwo or more second evaporation rates, the rotation speed and the two ormore horizontal cross-sectional areas are adjusted to form the unitlayer with a predetermined dopant concentration.
 43. The system of claim36, wherein one of the one or more first source materials is a dopantmaterial, wherein at least the one or more first evaporation rates, thetwo or more second evaporation rates, the rotation speed and the two ormore horizontal cross-sectional areas are adjusted to form the unitlayer with a predetermined dopant concentration.
 44. A method forfabricating a perovskite film by using a system of claim 35, the methodcomprising: controlling a temperature of the substrate stage forproviding uniform cooling or heating to the substrate; rotating thesubstrate stage at the rotation speed; controlling temperaturesassociated with the one or more evaporation units in the first set andthe two or more evaporation units in the second set to adjust the one ormore first evaporation rates and the two or more second evaporationrates, respectively; monitoring a film thickness in situ; andinterrupting the deposition when the film thickness reaches apredetermined thickness.
 45. The method of claim 44, further comprising:predetermining at least the one or more first evaporation rates, the twoor more second evaporation rates, the rotation speed and the two or morehorizontal cross-sectional areas to control composition and thickness ofa unit layer to fabricate the perovskite film comprising a plurality ofunit layers, wherein each unit layer is formed by one rotation of thesubstrate stage.
 46. The method of claim 44, wherein the system furthercomprises a shutter provided below the substrate stage, and a pluralityof evaporation shutters provided for the one or more evaporation unitsin the first set and for the two or more evaporation units in the secondset, respectively, the method further comprising: closing the shutter tocover the substrate stage before the deposition; opening the shutter toexpose the substrate stage to start the deposition; and adjusting theplurality of evaporation shutters to control flows of the vapors of thefirst and second source materials, respectively, wherein theinterrupting the deposition comprises closing the shutter.
 47. Aperovskite film fabricated by using the method of claim 44, wherein aroot mean square roughness per atomic force microscopy (AFM) is lessthan 40 nm without annealing.
 48. The system of claim 35, wherein the Ais an organic element selected from a group consisting of methylammonium(MA) and formamidinium (FA), the B is a metal element selected from agroup consisting of Pb and Sn, and the X is a halogen element selectedfrom a group consisting of Cl, I and Br.